Britain should train a "citizen army" ready to fight a war on land in the future, the head of the Army has said.
General Sir Patrick Sanders warned that an increase in reserve forces alone "would not be enough".
He highlighted the threat from Russia and pointed to steps being taken by other European nations to put their populations on a "war footing".
He also called for more to be done to equip and modernise the UK's armed forces.
In a speech at an armoured vehicle conference, Gen Sir Patrick was not making an argument for conscription - where men of fighting age are required to enlist in the military - but rather laying the foundations for a voluntary call up if war broke out.
He talked about the need for the UK's "pre-war generation" to prepare for the possibility of war and said that was a "whole-of-nation undertaking".
On Wednesday, the outgoing Chief of the General Staff (CGS) said Russia's war in Ukraine was about much more than seizing territory, saying it was about defeating our system and way of life.
He has already argued to reverse to recent cuts to the size of the Army. It is now a professional force of around 73,000, compared to around 100,000 in 2010.
In his speech he said Britain needed an army designed to expand rapidly.
"Within the next three years, it must be credible to talk of a British Army of 120,000, folding in our reserve and strategic reserve. But this is not enough," he said, as he also called for more to be done to modernise and equip the armed forces.
"We will not be immune and as the pre-war generation we must similarly prepare - and that is a whole-of-nation undertaking," he said. "Ukraine brutally illustrates that regular armies start wars; citizen armies win them.
"But we've been here before, and workforce alone does not create capability."
He highlighted steps being taken in countries like Sweden and Finland - where the threat of Russia looms closer - to put their nations more on a war footing.
Other senior Nato military commanders have also recently been calling on the alliance to ready itself for a potential conflict.
Such warnings make politicians nervous.
In response to Gen Sir Patrick speech the UK prime minister's spokesman said hypothetical scenarios of a future potential conflict were not helpful and ruled out any move towards a conscription model for the Army.
One senior Conservative MP told the OceanNewsUK he did not think Rishi Sunak had fully appreciated the threat posed by Russia.
The MP said that might be because the prime minister when growing up had not experienced the existential threat posed by the old Soviet Union during the Cold War era.
Gen Sir Patrick said the nation could not afford to make the same mistakes of 1914, when it failed to perceive the escalations that led to World War One.
He said over the last 30 years the Army had halved in size, with a 28% reduction in the last 12 years, but added that despite challenges in recruitment applications to join the Army were at the highest level in six years.
Gen Sir Patrick has been a vocal critic of cuts to troop numbers and military spending and will be replaced as CGS in June by General Sir Roly Walker.
He has not been the only one to criticise cuts, with former CGS General Lord Dannatt saying the UK risked a repeat of the 1930s unless it invested more in its armed forces last week.
